The invention relates to a sensor head for an X-ray detector and to an X-ray detector containing said sensor head, particularly for electron beam microanalysis.
Electron beam microanalysis is a widely used method of elementary analysis. This involves the detection and evaluation of X-ray radiation which is produced in an electron microscope, particularly a scanning electron microscope (SEM), when a sample is scanned by an electron beam. Having initially being planned only as a special application in conjunction with appliances designed specifically for the purpose, such as electron beam microprobes, electron beam microanalysis has since developed into a method which is routinely used in practically every SEM laboratory.
The definitive step therefor was the introduction of energy-dispersive X-ray detectors (EDX detectors), which are distinguished by their simple and robust design, low maintenance, stable operation and not least the relatively large captured solid angle.
EDX detectors have an input window, a semiconductor crystal, which is arranged axially in the visual axis of the detector on a “cooling finger”, and an amplifier unit, which typically comprises a field-effect transistor (FET) and a preamplifier. An EDX detector has a relatively large, homogeneous volume which can capture X-ray radiation from almost any direction. Particularly the large solid angle of the X-ray radiation captured by EDX detectors, i.e. the large amount of the radiation used from the total produced, allows microanalysis to be performed on ordinary, imaging electron microscopes.
The development of SEMs toward ever better electron-optical resolution, the improvement in electron detectors, but also the increased interest in organic or otherwise sensitive samples have resulted in the usual beam current being lowered to the extent that in many cases there are distinct limits set even with EDX detectors. It is currently possible to compensate for such limits only by virtue of extremely long measurement times.
The problem becomes particularly distinct in the case of “SDD detectors” (silicon drift detectors), the physical mode of action and design of which means that they would be able to capture a multiple of the usual volume of radiation. When the beam current is limited on account of the sample or the appliance, the only way of increasing the captured volume of radiation and hence reducing the measurement time is to increase the radiation solid angle captured by the detector. For this purpose, it is desirable for the distance between the detector and the sample to be as short as possible.
This is limited by the detector size and the design circumstances of the electron microscope, however. The best operating distance between pole shoe and sample for imaging production in the electron microscope is approximately 4 mm, with an increase in said operating distance impairing the image quality. In order to position the EDX detector as close to the sample as possible, the detector finger would optimally need to be positioned between pole shoe and sample in the electron microscope. There is therefore the need for the detector fingers of EDX detectors to be designed to be as small as possible so that the operating distance in the electron microscope does not need to be increased unnecessarily.
The sensor chip of the SDD detector must not be affected by this reduction, since otherwise the captured volume of radiation and hence the sensitivity of the SDD detector would likewise be reduced.
It is therefore the object of the present invention to minimize the detector size of an X-ray detector, particularly of an SDD detector, ideally while maintaining the sensor chip surface area.